The majority of commercially important hardwoods are notoriously difficult to bond. Poor adhesion of wood, especially hardwood, with adhesives and coatings is the major challenge for the acceptance of numerous timber species by wood product manufacturers.
It is commonly thought that the key reasons for poor adhesion of hardwoods and some softwoods are: interference of extractives and moisture, absence of durable chemical bonds between structural constituents of wood and adhesives or paints, and poor interlocking of adhesive with cell cavities. After machining, extractives rapidly migrate to the wood surface, creating a weak boundary layer which can seriously hinder adhesion. Deposits of extractives on the surface are thought to have the following adverse effects:                blocking the surface reactive sites;        incompatibility between the extractives and adhesives resulting in inferior bonding and hindering the cure of adhesives;        hindering surface wettability so that the surface wetting and/or penetration of particular adhesives into cellular structure of wood is adversely affected.        
Migration of extractives onto the wood surface also poses technical and logistic challenges to the wood product manufacturers, requiring wood product components to be freshly prepared and glued or coated quickly after machining.
Poor interfacial adhesion at the interface between the polymer and natural fibres in polymer composites is another problem due to the lack of adequate adhesion between the polymer matrix and the natural polymer fillers in form of fibre, chip or flour. This is often translating into poor performance of polymer-natural fibres composite and limiting its application.
Primer compositions and surface treatments are widely used to improve the bonding of coatings and adhesives with wood or wood products. They are used to provide good adhesion to the substrate surface, and in particular provide a satisfactory bond between the surface and the coating or adhesive and provide satisfactory surface stability until application of the coating or adhesive. Examples of commonly used primers include compositions based on marine or vegetable oils; alkyds and modified alkyds such as vinyl toluenated alkyds, styrenated alkyds, urethane alkyds, polyesters; cellulosic derivatives such as ethyl cellulose, nitrocellulose, cellulose acetobutyrate and modifications such as cellulose acrylics, cellulose alkyds and cellulose polyurethanes. Urethane series resin, melamine series resin and polyester series resin or lacquer is also used.
Latex type primers are also commonly used but in order to obtain the necessary durability and attractive appearance for exterior applications, it is recommended that the latex based primer be coated with two additional layers of latex paint. The latex primer improves adhesion to the substrate and helps to insure a uniform appearance of the top coat. Latex primers must be top coated because they are frequently discoloured by the tannins contained in the substrate. The latex primers function by letting the tannins migrate into the primer and then trapping the tannins within the primer layer. This prevents subsequent latex layers from being discoloured by the tannins. The additional layers of latex paint provide the durability to withstand weathering and provide a uniform, attractive appearance to the resulting coated substrate.
Primer composition and surface treatment method are often used to natural polymer fillers in form of fibre, chip or flour to improve the interfacial adhesion of the polymer composite and hence to improve their mechanical performance. Examples included dispersion or solution of maleated polypropylene and acrylic acid copolymers.
Paints or coatings are commonly used on wood products to prevent UV degradation or rotting or to provide an attractive surface finish. Surfaces of dry wood and wood-based products are, in general, electrically non-conductive so electrostatic painting or coating method is not applicable to them. Powder coating, using solvent-free solid resin, also requires conductive or semi-conductive surface and can not be applied to coat dry wood or wood-based products. Paints or coatings in the form of dispersion or solution are generally applied onto wood or wood-based products using traditional wet-spray method. Significant amounts of paint and coatings are wasted whilst using this method, especially for complex shape articles where uniform coating across the surfaces of interest is difficult to achieve. Electrostatic coating or powder coating using solvent free resin have advantages over traditional coating methods. Up to 80% less solvent-based paint or coating may be used in electrostatic painting process, while near-zero percentage of waste can be achieved whilst using powder coating process. Consequently, application of electrostatic painting or powder coating process can significantly reduce the VOC emissions and consequently help with the effective protection of environment.
Primer composition and treatment method capable of providing conductive or semi-conductive surface to wood or wood-based products to effectively facilitate the electrostatic painting or powder coating methods to these products would be advantageous to industry as a consequence of improving product quality, improving coating transfer efficiency as well as making the process more cost-effective and environment-friendly.
There is a need for a primer composition for improving the adhesion between a wood-based article and various materials such as coating materials and adhesives.
It is a further objective of the invention to provide a primer composition capable of providing the surface of wood or wood-based articles with good adhesion to adhesives or coatings. It would also be an advantage to have a primer capable of rendering wood conductive or semi-conductive to enable electrostatic painting or powder coating processes to be used with such articles.